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    SubjectRe: [PATCH v5 0/7] hugetlb_cgroup: Add hugetlb_cgroup reservation limits
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    On 9/27/19 3:51 PM, Mina Almasry wrote:
    > On Fri, Sep 27, 2019 at 2:59 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
    >>
    >> On 9/26/19 5:55 PM, Mina Almasry wrote:
    >>> Provided we keep the existing controller untouched, should the new
    >>> controller track:
    >>>
    >>> 1. only reservations, or
    >>> 2. both reservations and allocations for which no reservations exist
    >>> (such as the MAP_NORESERVE case)?
    >>>
    >>> I like the 'both' approach. Seems to me a counter like that would work
    >>> automatically regardless of whether the application is allocating
    >>> hugetlb memory with NORESERVE or not. NORESERVE allocations cannot cut
    >>> into reserved hugetlb pages, correct?
    >>
    >> Correct. One other easy way to allocate huge pages without reserves
    >> (that I know is used today) is via the fallocate system call.
    >>
    >>> If so, then applications that
    >>> allocate with NORESERVE will get sigbused when they hit their limit,
    >>> and applications that allocate without NORESERVE may get an error at
    >>> mmap time but will always be within their limits while they access the
    >>> mmap'd memory, correct?
    >>
    >> Correct. At page allocation time we can easily check to see if a reservation
    >> exists and not charge. For any specific page within a hugetlbfs file,
    >> a charge would happen at mmap time or allocation time.
    >>
    >> One exception (that I can think of) to this mmap(RESERVE) will not cause
    >> a SIGBUS rule is in the case of hole punch. If someone punches a hole in
    >> a file, not only do they remove pages associated with the file but the
    >> reservation information as well. Therefore, a subsequent fault will be
    >> the same as an allocation without reservation.
    >>
    >
    > I don't think it causes a sigbus. This is the scenario, right:
    >
    > 1. Make cgroup with limit X bytes.
    > 2. Task in cgroup mmaps a file with X bytes, causing the cgroup to get charged
    > 3. A hole of size Y is punched in the file, causing the cgroup to get
    > uncharged Y bytes.
    > 4. The task faults in memory from the hole, getting charged up to Y
    > bytes again. But they will be still within their limits.
    >
    > IIUC userspace only gets sigbus'd if the limit is lowered between
    > steps 3 and 4, and it's ok if it gets sigbus'd there in my opinion.

    You are correct. That was my mistake. I was still thinking of behavior
    for a reservation only cgroup model. It would behave as you describe
    above (no SIGBUS) for a combined reservation/allocate model.
    --
    Mike Kravetz

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